非形式谬误（PPT讲稿）Informal Fallacies

Informal fallacies

1 Informal Fallacies

Formal vs informal fallacies a fallacy is a defect in an argument other than its having false premises An informal fallacy is a defect in the content of an argument.(A formal fallacy is a defect in the structure of an argument. We have seen many valid rules of deduction Formal fallacies can be understood as a use of unacceptable rules

2 Formal Vs Informal Fallacies • A fallacy is a defect in an argument other than its having false premises. • An informal fallacy is a defect in the content of an argument. (A formal fallacy is a defect in the structure of an argument.) • We have seen many valid rules of deduction. Formal fallacies can be understood as a use of unacceptable rules

Examples of formal Fallacies 1. Affirming the consequent P→Q/Q∥/P

3 Examples of Formal Fallacies • 1. Affirming the consequent: • P  Q / Q // P

2. Denying the antecedent P→Q/~P∥~Q

4 • 2. Denying the antecedent • P  Q / ~P // ~Q

3. Commutation of conditionals P→Q∥Q→P

5 • 3. Commutation of conditionals: • P  Q // Q  P

4. Improper transposition P→Q∥P→~Q

6 • 4. Improper transposition: • P  Q // ~P  ~Q

5. Improper disjunctive syllogism PvQ/P∥^Q

7 • 5. Improper disjunctive syllogism: • P v Q / P // ~Q

Before discussing the formal fallacies concerning categorical syllogism, we should first learn what is meant by a distributed term If a categorical proposition tells us something about every member of a class referred by a term, the term is distributed in the proposition

8 • Before discussing the formal fallacies concerning categorical syllogism, we should first learn what is meant by “a distributed term”. • If a categorical proposition tells us something about every member of a class referred by a term, the term is distributed in the proposition

Consequently the following underlined terms are distributed in the propositions AllS are p No s are p Some s are p Some s are not p Now. we can discuss the formal fallacies of categorical syllogism

9 • Consequently the following underlined terms are distributed in the propositions: – All S are P. – No S are P. – Some S are P. – Some S are not P. • Now, we can discuss the formal fallacies of categorical syllogism

6. Undistributed middle Some p are m (some politicians are liars) Some M are s (some liars are thieves) Therefore, Some S are P. (some politicians are thieves

10 • 6. Undistributed middle: • Some P are M (Some politicians are liars) • Some M are S (Some liars are thieves) • Therefore, Some S are P. (Some politicians are thieves)